1. Field of the Invention
The present invention relates to a condenser microphone having an emitter-follower output circuit that can increase the maximum output level without a transformer.
2. Related Background Art
Power supply systems for condenser microphones and other microphones are set forth in the EIAJ standard (RC-8162A). The EIAJ standard relates to a phantom power source and defines three types of supply voltages (12 V, 24 V, and 48 V). In the case where a phantom power source is used for a condenser microphone, the maximum amplitude of the output from the condenser microphone is thus 48 VP-P).
In order to increase the output from the condenser microphone, a transformer may be provided in an output circuit thereof. If it is not suitable to include a transformer in the output circuit, a transformer-free output circuit is employed. An emitter-follower circuit is generally used in such a transformer-free output circuit in view of reduced output impedance.
A conventional condenser microphone including such a transformer-free output circuit is explained with reference to FIG. 4. In FIG. 4, a condenser microphone unit 1 has one output terminal connected to an impedance converter 2 and the other terminal connected to a negative terminal of a phantom power source 3.
The output circuit includes emitter-follower connected transistors Tr1 and Tr2. The base terminals of the transistors Tr1 and Tr2 are connected to the respective output terminals of the condenser microphone unit 1 through respective capacitors. The positive terminal of the phantom power source 3 that supplies power to the condenser microphone unit 1 is connected to the respective emitters of the transistors Tr1 and Tr2 through supply resistors R1 and R2. Thus, the supply resistors R1 and R2 also serve as load resistors for the emitter-follower connected transistors Tr1 and Tr2, respectively.
The respective emitters of the transistors Tr1 and Tr2 are connected to output terminals (not shown in the drawing). Audio signals converted by the microphone unit1 are balance-output from the output terminals. For a voltage of the phantom power source 3 of 48 V, the resistance value of the supply resistors R1 and R2 is defined as 6.8 kΩ and the deviation as within 0.4%.
For balanced output from the condenser microphone unit 1, the collectors of the emitter-follower circuits on a hot side (Tr1) and a cold side (Tr2) are coupled. The collectors of the transistors Tr1 and Tr2, which must be AC-grounded, are connected to the negative terminal of the phantom power source 3 through a capacitor (capacitor C3) having a relatively high voltage resistance and a large electrostatic capacitance.
As described above, the condenser microphone including the transformer-free conventional output circuit is provided with the two emitter-follower connected transistors Tr1 and Tr2. The voltage generated between the two ends of the large capacitor C3 serves as a power source to operate the impedance converter 2, the capacitor C3 connecting the coupled collectors of the transistors Tr1 and Tr2 to the negative terminal of the phantom power source 3.
The impedance converter 2 shown in FIG. 4 has a bias built-in FET including a biasing resistor and a diode.
The voltage supplied from the phantom power source 3 is divided into a voltage required to operate the emitter-follower circuits, which are included in the output circuit composed of the transistors Tr1 and Tr2, and a voltage required to operate the impedance converter 2 connected to the condenser microphone unit.
The maximum input sound pressure level of the condenser microphone is restricted mainly by the signal amplitude in the output circuit. Thus, it is desirable that the voltage to operate the emitter-follower circuits and the voltage to operate the impedance converter 2 connected to the condenser microphone unit be both high. Such a condenser microphone having a transformer-free output circuit is designed so as to set the two voltages to appropriate values.
A challenge of the conventional transformer-free output circuit is explained below in the case where the phantom power source 3 supplies 48 V power. The voltage between the two ends of each of the supply resistors R1 and R2 (each have a resistance value of 6.8 kΩ) of the phantom power source 3 and the voltage between the emitter terminal and the collector terminal of each of the emitter-follower connected transistors Tr1 and Tr2 functioning as the output circuit are substantially the same as the voltage between the terminals of the capacitor C3 (operating voltage for the impedance converter 3) connected to the collectors of the transistors Tr1 and Tr2. Thus, in the case where the voltage of the phantom power source 3 is 48 V, these voltages described above are each substantially 24 V. In the conventional output circuit shown in FIG. 4, the maximum output level is then 24 VP-P, and the maximum input sound pressure level is 18.7 dBV.
If an emitter-follower output circuit shown in FIG. 5 were to be used, the output level could be maximized. FIG. 5 is a diagram of a transformer-free output circuit of a condenser microphone, excluding an input side (a condenser microphone unit, for example). An emitter-follower circuit in FIG. 5 has a configuration similar to the emitter-follower circuit portion in FIG. 4. In the output circuit shown in FIG. 5, the voltage (48 V) supplied from the phantom power source 3 is divided into a voltage between two ends of each of supply resistors R1 and R2 and a voltage between an emitter terminal and a collector terminal of each of transistors Tr1 and Tr2.
Accordingly, the maximum output amplitude is substantially 24 V at each of output points C and D to which output terminals (not shown in the drawing) are connected in FIG. 5. In the output circuit in FIG. 5, the maximum output level is thus substantially 48 VP-P, and the maximum input sound pressure level is approximately 24.76 dBV.
Unlike the output circuit shown in FIG. 4, however, the power cannot be supplied to operate the impedance converter 2 (refer to FIG. 1), which is connected to the subsequent stage of the microphone unit 1, in the output circuit shown in FIG. 5. In other words, the impedance converter cannot be used in the output circuit of FIG. 5.
No prior art was found that has an object to solve the technical challenge described above, specifically to increase the maximum output level while maintaining the operating voltage for the impedance converter in the condenser microphone. Japanese Unexamined Patent Application Publication No. 2006-352622 relates to a condenser microphone having an emitter-follower connected transistor, for amplifying current, connected between an impedance converter included in a condenser microphone unit and an output transformer.